Honestly, since this is only in Colorado, it would be wise to look at other trends in Colorado the last 30 years. You could probably find some insight there.
Another question to ask would be “Has milk consumption decreased all over the US in the past 30 years?” The answer is yes.
I would bet milk consumption has decreased all over the US in the past 30 years as well.
Most likely, these two things are purely coincidental, both due to various cultural changes in Colorado, and maybe the entire US.
I know, I know, the P value.
The P value doesn’t measure whether or not the correlation is causal or even related. It really just measures the probability of falsely rejecting the null hypothesis. Basically, in this case, it says “Ues, these two things are correlated.”